Rhythmbox in addition to a plugin I wrote (http://code.google.com/p/rhythmote/). Ideal for using any web enabled device as remote control for your music.
It is free for Linux users since you can install the plugin directly to rhythmbox.
For Windows or Mac users there's RuckusDJ (http://www.ruckusdj.com), A virtual machine pre-configured to have everything working out of the box